The present invention relates generally to integrated circuit memory devices and, more particularly, to an apparatus and method for integrating nonvolatile memory capability within static random access memory (SRAM) devices.
A typical SRAM device includes an array of individual SRAM cells. Each SRAM cell is capable of storing a binary voltage value therein, which voltage value represents a logical data bit (e.g., “0” or “1”). One existing configuration for an SRAM cell includes a pair of cross-coupled devices such as inverters. With CMOS (complementary metal oxide semiconductor) technology, the inverters further include a pull-up PFET (p-channel) transistor connected to a complementary pull-down NFET (n-channel) transistor. The inverters, connected in a cross-coupled configuration, act as a latch that stores the data bit therein so long as power is supplied to the memory array. In a conventional six-transistor (6T) cell, a pair of access transistors or pass gates (when activated by a word line) selectively couples the inverters to a pair of complementary bit lines. Other SRAM cell designs may include a different number of transistors, e.g., 4T, 8T, etc.
As is the case with other types of volatile memories, data within a conventional SRAM is lost once power is removed or deactivated. In contrast, nonvolatile RAM devices retain the cell data when its power supply is turned off by utilizing a floating gate transistor having a charge placed thereon to modify the threshold voltage (Vt) of the device in a manner that reflects the state of the data retained in the cell. This type of device is well known in the art and may generally be classified according to three types of nonvolatile RAM: Erasable Programmable Read Only Memory (EPROM); Electrically Erasable Programmable Read Only Memory (EEPROM); and Flash memory that may be erased and programmed in blocks consisting of multiple locations.
Although the read performance of nonvolatile RAM (e.g., Flash) devices is somewhat adequate in terms of speed, the write operation of these devices is much slower (e.g., on the order of a few milliseconds) as compared to the nanosecond range of an SRAM device. Similarly, the power involved in a non-volatile read is comparable to that of an SRAM, however the power involved in a write operation is much greater for the non-volatile cell. Accordingly, it would be desirable to be able to combine the speed performance characteristics of an SRAM device with the non-volatility of floating gate devices, and in a manner that minimizes increases in device real estate so as to result in a so called “universal memory.”